The present invention relates to an image forming apparatus.
A commonly used image forming apparatus for printing a color image on both sides of a page of a book or the like is a tandem type duplex color image forming apparatus, wherein multiple image forming units, for forming monochromatic images on an image carrier, are arranged on both sides of continuous web (hereinafter referred to as a “web recording medium”). In this apparatus, the monochromatic image formed by each image forming unit is transferred onto an intermediate transfer member, or directly onto the web recording medium, whereby a color image is formed.
The above-described tandem type duplex color image forming apparatus is configured as shown in FIG. 1, for example. As seen in FIG. 1, on one side of the web recording medium, monochromatic image forming units for black (hereinafter referred to as “K”), yellow (hereinafter referred to as “Y”, magenta (hereinafter referred to as “M”) and cyan (hereinafter referred to as “C”) are arranged in sequence in the forward feed direction of the web recording medium. Similarly on the other side of the web recording medium, monochromatic image forming units for K, Y, M and C are arranged in sequence in the forward feed direction of the paper. These image forming units are arranged so that the path of the web takes a staggered form with image forming units alternating on both sides of the pap r, as is apparent at first glance.
In this image forming apparatus, four color toner images from four image forming units are overlapped on a transfer medium in the form of one web of recording medium, thereby forming a color image. However, when eccentricity due to installation conditions of various rotary bodies and eccentricity due to clearance error of the rotary shafts of these rotary bodies has occurred, image forming misregistration appears as color misregistration among the different color toner images. This will lead to deterioration of the image quality. Thus, to ensure high image quality, some measures must be taken to reduce such image forming misregistration.
To solve these problems, various proposals have been disclosed, which, for example, can be broadly classified into the following two types: one type is based on the technique of adjusting the rotary phase of the roller, and the other type is based on the technique of adjusting the rotary phase of the drum-shaped image carrier to a specified phase.
An example of the first technique, which involves adjusting the rotary phase of the roller, is disclosed in Japanese Application Patent Laid-Open Publication No. Hei 10-20604. The image forming apparatus disclosed in this publication is configured to ensure relative adjustment of the rotary phase of the image carrier of each image forming unit in order to avoid overlapping between peaks of the vibration components of periodic rotational changes that may be caused by roller eccentricity.
An example of the second technique, which involves adjusting the rotary phase of a drum to a specified phase, is disclosed in Japanese Application Patent Laid-Open Publication No. Hei 10-333398. The image forming apparatus disclosed in this publication is configured to detect the misregistration in mapping with respect to a point caused by an irregular speed of the drive system, including a belt and drum, and to ensure a specific relationship between the drum rotational position detected by a drum position sensor and the transfer position on the belt.
In the case of an image forming apparatus based on the above-described first technique, it is necessary to use a rotary body position sensor for detecting a color misregistration pattern or for reading a permeable marker having a high degree of permeability, for example, a CCD sensor with multiple photodetecting pixels arranged in a linear form using transmitted light, or a magnetically permeable belt position sensor.
Further, in the case of an image forming apparatus based on the above described second technique, some measures must be taken to prevent misregistration caused by eccentricity resulting from the staggered layout, since the image forming units are arranged on one side of the web of recording medium.
In the above-described first and second techniques, a pattern or marker is provided on the image carrier of the intermediate transfer member or the endless belt. This requires a rotary body position sensor for reading the pattern or marker, which fails to cut down costs and to improve productivity—a common problem in these types of apparatus.